As I discussed last month — most gravity distribution techniques have inherent flaws when it comes to how evenly they distribute effluent to the soil treatment area. As I travel across the country, I often get questions about the use of distribution boxes versus dropboxes as some regulations favor one over the other or do not allow one to be used versus another. My typical answer is that is a better conversation shared over an adult beverage as it is more opinion rather than fact. There are no studies that compare the treatment or longevity of distribution versus dropboxes. 

It is important to note that most of our regulations for conventional systems are based on research that assumes uniform distribution to the soil treatment interface. Before we get into the differences and advantages/disadvantages, I want to remind you that with septic tank effluent neither device provides uniform distribution to the soil until a biomat exists across the entire soil infiltrative surface. And even then, it is likely not 100% even due to soil variability.

Our goal with soil treatment systems is as even of distribution as possible to promote both treatment and acceptance of the effluent. This is critical from both a treatment and longevity standpoint.

With gravity distribution, a control structure of some type is very beneficial as it allows for active management of the system. They provide a means of locating the lines, making flow adjustments, monitoring the system and potentially adding onto the system.

Dropboxes

When a dropbox is used the distribution is serial, meaning at system startup, only one trench is used until the effluent loads the dropbox up to the point that some of the effluent is diverted to the second trench. In the dropbox there is an outlet near the bottom of the dropbox, often connected to the distribution pipe of the trench. Another outlet near the top of the dropbox either leads to another section of the trench going in the opposite direction or to the dropbox of the lower trench.

The effluent only goes to the second trench when the first trench is accepting effluent at its long-term acceptance rate (not in a state of malfunction) and fully ponded if installed correctly. If the elevation of the bottom of the supply pipe is equal to the top of the distribution media in the trench, this will achieve a liquid level that will maximize the trench sidewall, develop the maximum hydraulic head on the bottom of the trench and maximize the potential for evapotranspiration during the growing season.

With a dropbox, only the portion of the soil treatment unit required to treat the effluent is used.

Serial distribution encourages biomat formation and unsaturated flow to form as quickly as possible in the first trench as all the septic tank effluent with its organic material, solids and nutrients is directed into one line of the trench. In faster percolating soils (think any soil texture ending in sand) this “quicker” development of a biomat assists with the treatment process.   Aside from the order in which effluent reaches them, the trenches function independently, each receiving effluent at the rate it is accepted in that trench. If one is draining more slowly than the others — perhaps because it is in less permeable soil — it will accept less effluent. If one tends to drain quickly — perhaps because it receives more sunlight on the surface and more water is lost through evaporation in the warmer months — it will receive more effluent.

Since the trenches are not directly connected, there is no hydraulic head from trench to trench — effluent does not move more quickly into or through the second or third trenches because they are downhill from the first one.

Due to the “drop” in dropboxes, they are most suitable for sloping sites but can be used on level sites by positioning the downstream boxes two inches lower than the up-slope unit (if soil conditions allow this). The first inch is for the elevation difference between the inlet pipe and the supply pipe to the next dropbox, and the second inch is for the slope of the supply pipe to the next dropbox.

The dropbox distribution system allows for flexibility. If additional soil treatment system capacity becomes necessary, additional trenches can be added. This system can also be constructed on steeper slopes than other distribution methods. Each trench can be any length to accommodate structures and trees if the total trench length is adequate for the wastewater source and site conditions.

Distribution Boxes

Distribution boxes utilize parallel distribution of the septic tank effluent by gravity to equally load all sections of the soil treatment and dispersal system. Parallel distribution directs effluent flow into all trenches in the soil treatment unit simultaneously. Trenches are constructed to be of equal length and depth and to be suited for the same type of soil so treatment occurs at the same rate in each trench. In theory, this allows for equal use of all parts of the system. However, in practice, this rarely happens as trenches are not identical to each other.

All the outlets of the distribution box must be at the same elevation when installed, after the system has been backfilled, and years after use. Typically, a proprietary weir is placed on each outlet to adjust and equalize flow. 

When you have limited appropriate soil prior to a restriction such as redoximorphic features or bedrock, a distribution box may be needed to avoid loss in elevation between trenches needed with dropboxes.

Installation Keys

As was described, evaluations and levelness of components are critical with both distribution and dropboxes. They must be properly bedded to remain stable over time. The proper bedding could be a layer of washed rock or pea gravel placed on a level foundation of undisturbed or properly compacted native soil. Alternatively, they can be placed directly on smooth, level undisturbed native soil.

Pipes exiting the device must be securely installed and properly bedded or they will settle over time resulting in uneven distribution, infiltration or both. Bedding pipes connected to the boxes must be done carefully to avoid damaging the box or causing wall deflection. This work should be performed by hand because machine traffic can potentially crush components.

An inspection port on the end of the trench is also beneficial for measuring ponding of each line. With a dropbox system, this can help confirm how many trenches are in operation and at what capacity, and with a distribution box you can assess the evenness of the distribution. Both types of boxes should be installed with access to grade for management. 

Management

Serial distribution systems with dropboxes can be rested. After a year or more of use, you can cap the outlet pipe from the first box. The first trench will rest, fully drying out with the natural decomposition of residual organic matter. 

With distribution boxes, maintenance and adjustment is critical to assure equalization of flow to the trenches due to the settling of the distribution box and piping. If solids or other debris reach the box, it must be cleaned or flow to the trenches will be impacted. If debris, solids or FOG builds up in the box, it should be cleaned or the distribution may be impacted.

Summary

Although many regulations, installers and designers have a preference on serial versus parallel distribution, a study has never been conducted comparing the longevity of the two systems. The choice between them should be based on site conditions and system management oversight.